Background Hereditary spastic paraplegias (HSP) are neurologic disorders characterized by progressive lower‐extremity spasticity. Despite the identification of several HSP‐related genes, many patients lack a genetic diagnosis. Objectives The aims were to confirm the pathogenic role of biallelic COQ4 mutations in HSP and elucidate the clinical, genetic, and functional molecular features of COQ4‐associated HSP. Methods Whole exome sequences of 310 index patients with HSP of unknown cause from three distinct populations were analyzed to identify potential HSP causal genes. Clinical data obtained from patients harboring candidate causal mutations were examined. Functional characterization of COQ4 variants was performed using bioinformatic tools, single‐cell RNA sequencing, biochemical assays in cell lines, primary fibroblasts, induced pluripotent stem cell–derived pyramidal neurons, and zebrafish. Results Compound heterozygous variants in COQ4, which cosegregated with HSP in pedigrees, were identified in 7 patients from six unrelated families. Patients from four of the six families presented with pure HSP, whereas probands of the other two families exhibited complicated HSP with epilepsy or with cerebellar ataxia. In patient‐derived fibroblasts and COQ4 knockout complementation lines, stable expression of these missense variants exerted loss‐of‐function effects, including mitochondrial reactive oxygen species accumulation, decreased mitochondrial membrane potential, and lower ubiquinone biosynthesis. Whereas differentiated pyramidal neurons expressed high COQ4 levels, coq4 knockdown zebrafish displayed severe motor dysfunction, reflecting motor neuron dysregulation. Conclusions Our study confirms that loss‐of‐function, compound heterozygous, pathogenic COQ4 variants are causal for autosomal recessive pure and complicated HSP. Moreover, reduced COQ4 levels attributable to variants correspond with decreased ubiquinone biosynthesis, impaired mitochondrial function, and higher phenotypic disease severity. © 2023 International Parkinson and Movement Disorder Society. Pathogenic mutations in COQ4 were identified in six unrelated families with hereditary spastic paraplegia. COQ4 loss of function reduced cell proliferation, increased mitochondrial reactive oxygen species, and decreased mitochondrial membrane potential; these collectively contribute to the observed reduction in coenzyme Q10 levels. COQ4 mRNA is enriched in pyramidal neurons, and coq4 knockdown in zebrafish caused mobility defects.